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Today’s ObjectivesToday’s Objectives

Announcements• Homework #3 is due on Monday, 10-Jul, however you can earn 10 bonus

points for this HW if you turn it in on Wednesday, 5-Jul• Link to a grade estimator is in the Announcements of our class page• Last day to drop a 9-week class without grade penalty is 5-Jul.

To drop, contact UHCL directly

Progress review Using string and stringstream classes Operator Overloading (Ch. 11)

• Fundamentals of operator overloading• Overloading binary and unary operators• Overloading stream insertion and stream extraction operators• Overloading ++ and --

3-Jul-20063-Jul-2006

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Progress ReviewProgress Review

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So far, we have learned…So far, we have learned…

Object-Oriented Programming• Basics of C++• Functions and argument passing• Arrays• Pointers and dynamic memory allocation• C++ classes and objects• Some C++ standard libraries and their namespace• The STL vector class• Programs that use objects to solve problems• Debugging techniques• Encapsulation• Information hiding

Object-Oriented Analysis and Design• A simple software process• Pseudocode algorithms• UML class diagrams and use case diagrams

Progress ReviewProgress Review

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Next…Next…

The string class and the stringstream class Operator overloading Inheritance Polymorphism Templates Stream I/O Exception handling File processing Linked lists The g++ compiler

Progress ReviewProgress Review

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Operator OverloadingOperator Overloading

Chapter 11

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Operators are Like FunctionsOperators are Like Functions

With objects, an operator, like “+”, is really just a function with a different syntax

With a binary operator, the arguments are placed on either side of the operatorstring air = "air", plane = "plane";string combined = air + plane;

The compiler generates the following function callstring combined = operator+( air, plane );

Operator Overloading (Deitel, 572–621; Savitch) Operator Overloading (Deitel, 572–621; Savitch)

lhs rhs

lhs rhs

operator

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Operators Can Be OverloadedOperators Can Be Overloaded

Just like any function, operators can be overloaded

Operators are already overloaded so that they “do the right thing” with all built-in data types, like int and double

42 + 58;

98.6 + 0.3;

Also with classes in the standard library, like string

string result;result = result + "test";

But operators are not automatically overloaded for classes that we create

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

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Operators Can Be OverloadedOperators Can Be Overloaded

We have to overload operators for the classes that we create

The implementation of an overloaded operator is usually a member function of the class of the object used on the lhs

Example of a binary operator used with objects of a class we madeBook book1, book2; //Instantiate two book objectsBook book3 = book1 + book2;

The operator + must be implemented as a public member functionclass Book{public: Book& operator+( const Book& rhs );//remainder of the code left out...};

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

lhs

rhsoperatorlhs

rhs

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Operators that Can Be OverloadedOperators that Can Be Overloaded

Fig. 11.1, page 574

One operator that never has to be overloaded• operator&

– The “address of” operator– Can be used with any object to get its address

One operator that has to be overloaded only when a data member is created with “new”• operator=

– Assignment operator– Can be used with any objects for default memberwise

assignment– Must be overloaded when default memberwise assignment

doesn’t work correctly

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

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RestrictionsRestrictions

Precedence of operators cannot be changed

Arity of operators cannot be changed (“arity” is the number of operands)

New operators cannot be created

Operators for built-in data types cannot be overloaded

Some operators cannot be overloaded

• Fig. 11.2, page 574

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

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Sample Class in Book.hSample Class in Book.h

#ifndef BOOK_H#define BOOK_H#include <string>using std::string;

class Book{public:

Book( string t="",int cpy=0 ): title(t), copies(cpy){}void setTitle( string t ){ title = t; }string getTitle(){ return title; }void setCopies ( int cpy ){ copies = cpy; }int getCopies(){ return copies; }string toString(){ return "Book: " + title; }

private:string title;int copies;

};#endif

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

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Binary OperatorsBinary Operators

Operands are placed on both sides of the operators Example: the equality operator

• UsageBook book1("C++ How to Program",1);Book book2("Java How to Program",1);if( book1 == book3 ) cout << "Equal" << endl;

• Implementationclass Book{

public: bool operator==( const Book& rhs ){

return this->title==rhs.title; }

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

lhs rhsoperator

lhs

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Unary OperatorsUnary Operators

Only one operand Example: the not operator

• UsageBook book3;if( !book3 ) cout << "empty" << endl;

• Implementationclass Book{public:

bool operator!(){return ( title=="" && copies==0 );

}

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

lhs

The conditions of an “empty” object

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Stream Insertion OperatorStream Insertion Operator

The stream insertion operator is a binary operator Usage

cout << "Hello";

The lhs is a C++ object of the ostream class which is in the iostream library

The ostream class knows how to output any of the built-in C++ data types and any standard library data types

We have to overload << for our own classes, but we can’t put the overloaded operator into the lhs class

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

lhs rhsoperator

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Stream Insertion OperatorStream Insertion Operator

We implement << as a global function and then declare it as a friend of our own class

Example:• Usage

cout << book1;

• Implementation

class Book{friend ostream& operator<<( ostream& out, const Book& rhs );//remainder of the Book class here};

Global function definition:

ostream& operator<<( ostream& out, const Book& rhs ){out << rhs.title;return out;

}

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

lhs rhs

A friend has access to the private data members

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Stream Extraction OperatorStream Extraction Operator

Implement as a friend function Example:

• Usagecin >> book3; //Already declared like this: Book book3;

• Implementation

class Book{friend istream& operator>>( istream& in, Book& rhs );//remainder of the Book class here};

Global function definition:

istream& operator>>( istream& in, Book& rhs ){in >> rhs.title;//captures a single word, no whitespacein >> rhs.copies;return in;

}

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

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Overloading ++Overloading ++

Special syntax to distinguish pre- from post-increment Example:

• Usagebook1++;cout << "Copies = " << book1.getCopies() << endl;

• Implementation as member functions of the Book classBook& operator++(){ //Pre-increment

++copies;return *this;

}

Book operator++(int){ //Post-incrementBook temp = *this;++copies;return temp;

}

Operator Overloading (Deitel, 572–621) Operator Overloading (Deitel, 572–621)

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Library DemoLibrary Demo

(Continued)

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Library DemoLibrary Demo

Progress review• Implemented two classes in the first version

– Book class– BookList class– Library class

TODO• Add to the Book class

– operator>>– operator<<– operator==

• Use these three operators of the Book class– Use >> in main to get the input for a new book– Use << in the printList member function of BookList– Add a find member function to BookList and use ==

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ReferencesReferences

Deitel, H. M., and P. J. Deitel, C++ How to Program, Fourth Edition. Upper Saddle River, NJ: Prentice Hall, 2003.

Lippman, Stanley B., and Josee Lajoie, C++ Primer. Boston: Addison-Wesley, 1998.

Savitch, W., Problem Solving with C++, Fifth Edition. Boston: Addison-Wesley, 2005.